Electrochromic devices are devices in which a physical or chemical change produced in response to an applied voltage results in a change in the reflective or transmissive properties of the device with respect to electromagnetic radiations, e.g., uv, visible, or IR radiations. Electrochromic materials are those materials which change coloration in an electrochromic device. Most electrochromic devices are multilayer devices containing an electrochromic electrode layer, an ion conductor layer, and a counter electrode layer. The counter electrode layer may or may not be electrochromic.
Many United States Patents have been granted for multilayer electrochromic devices see for example U.S. Pat. Nos. 4,175,838, 4,231,641, 4,335,938, 4,338,000, 4,375,318 and 4,573,768. U.S. Pat. No. 3,560,078 disclosed a multilayer electrochromic device not having an ion conductor layer, i.e., the electrochromic layer was in contact with the counter electrode layer. In any event, each electrode layer generally has a current carrier by which current and voltage are applied. For transmissive devices this current carrier is frequently a thin layer of a metal such as gold or platinum, or a thicker layer of an oxide such as tin doped indium oxide (ITO). A polymeric electrochromic electrode is an electrochromic electrode that incorporates an electrochromic material in a polymeric matrix.
Tungsten oxide (WO.sub.3) is a highly preferred electrochromic material for electrochromic devices because of its excellent optical, chemical and physical properties. It is, perhaps, the most well studied and often used of all electrochromic materials. Tungsten oxide is often deposited as a layer in a multilayer electrochromic device by vacuum evaporation techniques, specifically DC and RF reactive sputtering from either the metallic tungsten or the oxide. Tungsten oxide can also be deposited as a layer in a multilayer electrochromic device by resistive or electron-beam thermal vacuum evaporation techniques. These vacuum deposition techniques require relatively expensive equipment. Therefore, it would be an advance in the art of electrochromic devices if a less expensive fabrication approach could be found.
As stated in Cotton and Wilkinson Advanced Inorganic Chemistry (1980) on page 852: "Mo.sup.6+, W.sup.6+, V.sup.5+, Nb.sup.5+, and U.sup.6+ are known to form poly acids." "The poly acids of molybdenum and tungsten are of two types: (a) the isopoly acids [e.g., H.sub.2 W.sub.12 O.sub.40.sup.6- ] and their related anions, which contain only molybdenum or tungsten [vanadium, niobium or uranium] along with oxygen and hydrogen, and (b) the heteropoly acids [e.g., PW.sub.12 O.sub.40.sup.3- ] and their related anions, which contain one or two atoms of another element in addition to molybdenum or tungsten, [vanadium, niobium or uranium] oxygen and hydrogen." A subgrouping of heteropoly acids are the transition metal substituted heteropoly acids in which one transition metal atom, e.g., one tungsten atom, is replaced by a different transition metal atom (usually of the first transition series) . Examples of this are PCo.sub.2 W.sub.11 O.sub.40 H.sub.2.sup.5-, SiCoMo.sub.11 O.sub.40 H.sub.2.sup.6-, Co.sub.2 W.sub.12 O.sub.40.sup.6-, and SiFe.sub.3 W.sub.11 O.sub.39.sup.5-. By definition herein, the term polyoxometallate includes all of the above isopoly acids and their salts, heteropoly acids and their salts, transition metal substituted isopoly acids and their salts and transition metal substituted heteropoly acids and their salts.
Most polytungstates and other polyoxometallates are known to be electrochromically active. For example: U.S. Pat. No. 3,283,656 issued to Gif Jones and Ralph Friedrich on Nov. 8, 1966 disclosed the use of polyvanadates, polytungstates, or polymolybdates in a one compartment liquid filled device, wherein the liquid contained the polyvanadate, polymolybdate, or polytungstate; and U.S. Pat. No. 3,560,078 issued to James Mcintyre and Robert Hansen on Feb. 2, 1971 disclosed the use of polytungstic and polymolybdic acids in a solid state multilayer device.
Schimidzu, et. al., 84 J. Chem. Soc., Faraday Trans. 1, 1988, page 3941, taught the use of a polyoxometallate in an electroactive and intrinsically electronically conducting polymer matrix such as a polypyrrole or polythiophene. However, in this system the polyoxometallate is released from the matrix upon application of some of the more cathodic coloring potentials. In addition, the electrode of Schmidzu, et. al. does not have a colorless state.
Keita, et. al., 279 J. Electroanal. Chem. 1990, 187, taught the use of polyoxometallates entrapped in an electronically conducting ion exchange polymer matrices coated onto carbon electrodes for purposes of catalysis of the hydrogen evolution reaction. However, Keita, et. al. is silent about the use of such electrodes applied to transparent electronic conductors for use in electrochromic devices and Keita, et al.'s electrode is immersed into a solution.